Ceramic materials possess properties such as high strength, high hardness, high stiffness, corrosion resistance, low coefficients of thermal expansivity, and lower density than most metals, which make these materials highly desirable for structural applications. However, ceramic materials also inherently possess low fracture resistance. As such, they are usually reinforced to enhance fracture resistance for suitability in structural applications. Various methods have been employed for reinforcing ceramic structures.
Conventional techniques such as particle reinforcement, laminated structures, co-continuous composites, and functional gradient methods, have been employed to enhance the mechanical resistance of ceramic materials. However, these methods either exhibit trade-offs between fracture strength and fracture toughness or are not amendable to proper control to achieve the desired end product properties.
For example, interpenetrating composites (IPCs), or co-continuous composites, may be fabricated by infiltrating molten metallic phase into porous preforms, as described in F. Wagner, et al., “Interpenetrating Al2O3—TiAl3 Alloys Produced by Reactive Infiltration,” J. Eur. Ceram. Soc. 19, 2449-53 (1999). Reactive melt infiltration has also been used to infiltrate porous preforms, as described in C. San Marchi, et al., “Alumina-aluminum interpenetrating-phase composites with three-dimensional periodic architecture”, Scripta Materialia 49, 861-66 (2003). In these methods, the reinforcement phase is expected to fill the pores and channels of the preform to form a complete composite. Accordingly, due to the anticipated interconnectivity of the phases, IPCs are expected to exhibit higher mechanical resistance than other reinforcements. However, due to present limitations, property enhancements resulting from these methods fail to meet expectations. For example, during reactive infiltration, the reaction may not be complete, thereby resulting in residues which alter material properties. Additionally, the ceramic pore structure may be characterized by lack of pore connectivity and regularity, as is common in present methods of fabrication of porous ceramics.
For example, partial sintering of green bodies is a commonly used method of forming porous ceramics, but results in a high percentage of closed pores, as described in Dale Hardy & David J. Green, “Mechanical Properties of a Partially Sintered Alumina,” J. Eur. Ceram. Soc. 15, 769-75 (1995) and A. Mattern, et al., “Preparation of interpenetrating ceramic-metal composites,” J. Eur. Ceram. Soc. 24, 3399-408 (2004). Other techniques for fabricating porous ceramics include the replica method, invented by Schwartzwalder and Somers in 1963, the sacrificial template method, direct foaming, freeze casting, and gel casting, as described in Andre R. Studart, et al., “Processing Routes to Macroporous Ceramics: A Review,” J. Am. Ceram. Soc. 89 [6], 1771-89 (2006); Urs T. Gonzenbach, et al., “Macroporous Ceramics from Particle-Stabilized Wet Foams,” J. Am. Ceram. Soc. 90 [1], 16-22 (2007); Byung-Ho Yoon, et al., “Aligned porous alumina ceramics with high compressive strengths for bone tissue engineering,” Scripta Materialia 58, 537-40 (2008); and P. Sepulveda and J. G. P. Binner, “Processing of Cellular Ceramics by Foaming and in situ Polymerization of Organic Monomers,” J. Eur. Ceram. Soc. 19, 2059-66 (1999).
Porous ceramics fabricated by these methods are more suitable for purposes of filtration, insulation, catalyst support, and the like. These ceramics are not very effective preforms for reinforcement purposes, due to the characteristics of their cellular structures. Moreover, conventional open foams are characterized by structures of cells interconnected by randomly distributed necks of varying sizes and shapes. Thus, composites made from such preforms may display randomly distributed weak regions. These weak regions result in a weakening of the overall mechanical properties of the material and an undesirable increase in the variability of other material properties.
It therefore would be desirable to provide improved methods of forming ceramic materials for use in structural applications. It would also be desirable to provide ceramic materials having improved properties, for example, to render them more suitable for use in various structural applications.